Abstract

Heidrun, the first deep draft concrete TLP, was installed in July 1995 in a water depth of 345m on the Haltenbank of the Norwegian Continental Shelf. The TLP has a displacement of 290610 tomes and the hull was designed to support a total topsides weight of 64980 tomes on Module Support Beams (MSBS) weighing 18550 tonnes.

The TLP is a marine and structural system with complex interaction between the hull, topsides and the mooring system. This paper describe a the basis for the sizing of the hull and tether system. Key issues addressed include weight control, stability during installation, tether system loads and constraints from fabrication and installation. The relative merits of the use of concrete rather than steel for the hull construction are discussed from a hydrodynamic perspective.

Hydrodynamic model testing was used to resolve many critical design issues, including 'ringing' which has had significant consequences for other deep water offshore platforms. New criteria were established for tether pretension.

The Operational Expense of TLP Marine Control has been minimized by provision of the Marine Control and Data Acquisition system (MCDA), which is used for in-service control of tether tension and for acquisition of key environmental and performance data. Continuity of weight control during Engineering, Construction, Installation and Operation was successfully implemented. TLP Performance data is presented in sea states up to 10.5m significant.

Evaluation' by the Conoco Project Team of TLP performance, on-board training of Operators and provision of an automatic system for continuous shore-base monitoring of marine performance facilitated the timely handover of responsibility for TLP Marine Control to Statoil Operations.

Introduction

Early conceptual studies [1, 2] identified several advantages of the concrete hull which were key in concept selection. These include elimination of fatigue concerns at complex node intersections, high distance to impact loads, relatively simple and faster construction, low sensitivity to increases in topside a weight and low tether design loads.

Preliminary Engineering Contracts were awarded to two competing Contractors in February 1991 for development of a concrete hull TLP for the Heidrun Field Development. The Engineering, Procurement, Construction and Installation (EPCI) contract was awarded to Norwegian Contractors (NC) in December91. Following wind tunnel testing of the topsides in November91, the NC concept was model tested in January 1992 in order to accommodate the design freeze scheduled for the end of March 92.

Unexpectedly high loads were measured in the tether system due to 'ringing', the term given to the global vibration of the TLP at it a natural period in pitch and roll, coined during the model testing of the Hutton TLP in 1979-80. Ringing was previously considered for steel TLPs to be a minor load effect and its cause had never been clearly identified.

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